1. Field of the Invention
The present invention relates to a fire-retardant adhesive containing a non-halogen flame-retardant with excellent insulating properties, a fire-retardant adhesive film and a flat cable that use the fire-retardant adhesive.
2. Description of the Related Art
Known are Flat cables with a structure where several conductors in lines are covered with 2 pieces of adhesive film where an adhesive layer has been formed on a base film. In recent years this type of flat cable has come to be widely used in AV equipments and computer equipments packaged in a high density.
However, the structural materials other than the conductors of flat cables (that is, the adhesive film comprised of a base film and an adhesive layer) must show good insulating properties and good fire-retardant properties in terms of their electrical use and fire prevention.
Therefore, for years halogen flame-retardants such as decabromodiphenyl ether and hexabromodiphenyl ether and auxiliary flame-retardants such as inorganic fillers (for example, silica, clay and antimony compounds) have been added to the adhesive layer of an adhesive film used in flat cables. However, non-halogen flame-retardants have recently come to be used in place of halogen flame-retardants as the flame-retardant added to adhesive layers in view of the global environmental protection. For instance, Japanese Patent Application Laid-Open No. 6-338225 discloses the addition of a non-halogen flame-retardant such as red phosphorus, phosphoric acid ester and magnesium hydroxide to the polyester fire-retardant layer of adhesive tape for flat cables. Moreover, Japanese Patent Application Laid-Open No. 9-221642 discloses that a thermoplastic polyester resin base as an adhesive film for flat cables or the adhesive layer formed on the base is made fire-retardant by using a phosphorus flame-retardant.
However, when flat cables are used as the wiring for various products related to human safety such as the air bags in automobiles and medical equipments, they must have extremely high insulating properties (in concrete terms, at least 1010 xcexa9) in order to guarantee safety.
However, there are problems with the adhesive tape and the adhesive film disclosed in the above-mentioned patent documents in that the electrical resistance of the adhesive tape or the adhesive film cannot be increased enough because of the hygroscopicity or ionicity that the non-halogen flame-retardant that is used itself displays. Moreover, when compared to conventional halogen flame-retardants, the above-mentioned non-halogen flame-retardants intrinsically do not have sufficient fire-retardant properties and the non-halogen flame-retardant content of the adhesive layer must be increased in order to realize the desired insulating properties. Therefore, there is a problem in that the adhesive strength of the adhesive layer lowers and hence the adhesive layer and conductors readily peel apart when the flat cable is produced, or the bonding force of the adhesive layer lowers.
A object of the present invention is to solve the problems of prior art, and also to give fire-retardant properties rivaling that of halogen flame-retardants to fire-retardant adhesives and fire-retardant adhesive films that use a non-halogen flame-retardant and are useful for the production of fire retardant flat cables while retaining good electrical insulating properties and good adhesion.
The inventors successfully completed the present invention upon discovering that the above-mentioned object can be accomplished by using a polyester resin with relatively low hygroscopicity as the base resin for the fire-retardant adhesive and by using a nitrogen-containing organic flame-retardant (e.g. melamine) as the flame-retardant together with a boron compound as the auxiliary flame-retardant.
That is, the present invention provides a fire-retardant adhesive, comprising the following components (A), (B), and (C):
(A) a polyester resin;
(B) a nitrogen-containing organic flame-retardant; and
(C) a boron compound.
Moreover, the present invention provides a fire-retardant adhesive film, comprising an insulating base film and provided thereon a fire-retardant adhesive layer comprised of the above fire-retardant adhesive.
Furthermore, the present invention provides a flat cable in which an electrical conductor is covered with the fire-retardant adhesive film.